Americans have been getting fatter for years, and with the increase in waistlines has come a surplus of conventional wisdom. If we could just return to traditional diets, if we just walk for 20 minutes a day, exercise gurus and government officials maintain, America’s excess pounds would slowly but surely melt away.

Scientists are less sanguine. Many of the so-called facts about obesity, they say, amount to speculation or oversimplification of the medical evidence. Diet and exercise do matter, they now know, but these environmental influences alone do not determine an individual’s weight. Body composition also is dictated by DNA and monitored by the brain. Bypassing these physical systems is not just a matter of willpower.

More than 66 percent of Americans are overweight or obese, according to the federal Centers for Disease Control and Prevention, in Atlanta. Although the number of obese women in the United States appears to be holding steady at 33 percent, for most Americans the risk is growing. The nation’s poor diet has long been the scapegoat. There have been proposals to put warning labels on sodas like those on cigarettes. There are calls to ban junk foods from schools. New York and other cities now require restaurants to disclose calorie information on their menus.

But the notion that Americans ever ate well is suspect. In 1966, when Americans were still comparatively thin, more than two billion hamburgers already had been sold in McDonald’s restaurants, noted Dr. Barry Glassner, a sociology professor at the University of Southern California. The recent rise in obesity may have more to do with our increasingly sedentary lifestyles than with the quality of our diets.

“The meals we romanticize in the past somehow leave out the reality of what people were eating,” he said. “The average meal had whole milk and ended with pie.... The typical meal had plenty of fat and calories.”

“Nostalgia is going to get us nowhere,” he added.

Neither will wishful misconceptions about the efficacy of exercise. First, the federal government told Americans to exercise for half an hour a day. Then, dietary guidelines issued in 2005 changed the advice, recommending 60 to 90 minutes of moderate exercise a day. There was an uproar; many said the goal was unrealistic for Americans. But for many scientists, the more pertinent question was whether such an exercise program would really help people lose weight.

The leisurely after-dinner walk may be pleasant, and it may be better than another night parked in front of the television. But modest exercise of this sort may not do much to reduce weight, evidence suggests.

“People don’t know that a 20-minute walk burns about 100 calories,” said Dr. Madelyn Fernstrom, director of the weight-management center at the University of Pittsburgh Medical Center. “People always overestimate the calories consumed in exercise, and underestimate the calories in food they are eating.”

Tweaking the balance is far more difficult than most people imagine, said Dr. Jeffrey Friedman, an obesity researcher at Rockefeller University. The math ought to work this way: There are 3,500 calories in a pound. If you subtract 100 calories per day by walking for 20 minutes, you ought to lose a pound every 35 days. Right?

Wrong. First, it’s difficult for an individual to hold calorie intake to a precise amount from day to day. Meals at home and in restaurants vary in size and composition; the nutrition labels on purchased foods — the best guide to calorie content — are at best rough estimates. Calorie counting is therefore an imprecise art.

Second, scientists recently have come to understand that the brain exerts astonishing control over body composition and how much individuals eat. “There are physiological mechanisms that keep us from losing weight,” said Dr. Matthew W. Gilman, the director of the obesity prevention program at Harvard Medical School/Pilgrim Health Care.

Scientists now believe that each individual has a genetically determined weight range spanning perhaps 30 pounds. Those who force their weight below nature’s preassigned levels become hungrier and eat more; several studies also show that their metabolisms slow in a variety of ways as the body tries to conserve energy and regain weight. People trying to exceed their weight range face the opposite situation: eating becomes unappealing, and their metabolisms shift into high gear.

The body’s determination to maintain its composition is why a person can skip a meal, or even fast for short periods, without losing weight. It’s also why burning an extra 100 calories a day will not alter the verdict on the bathroom scales. Struggling against the brain’s innate calorie counters, even strong-willed dieters make up for calories lost on one day with a few extra bites on the next. And they never realize it. “The system operates with 99.6 percent precision,” Dr. Friedman said.

The temptations of our environment — the sedentary living, the ready supply of rich food — may not be entirely to blame for rising obesity rates. In fact, new research suggests that the environment that most strongly influences body composition may be the very first one anybody experiences: the womb.

According to several animal studies, conditions during pregnancy, including the mother’s diet, may determine how fat the offspring are as adults. Human studies have shown that women who eat little in pregnancy, surprisingly, more often have children who grow into fat adults. More than a dozen studies have found that children are more likely to be fat if their mothers smoke during pregnancy.

The research is just beginning, true, but already it has upended some hoary myths about dieting. The body establishes its optimal weight early on, perhaps even before birth, and defends it vigorously through adulthood. As a result, weight control is difficult for most of us. And obesity, the terrible new epidemic of the developed world, is almost impossible to cure.